CMOS image sensors are a kind of solid state image sensors. A CMOS image sensor with a resolution of M×N generally includes an image sensor array (with a size of M×N), an analog readout processing circuit, and a digital control module etc. Among them, the analog readout processing circuit has a size of 1×P, where 1≤P≤N; wherein when P=1, it performs serial readout, has the lowest degree of parallelism, and is mainly used for a low-speed image sensor; and when P=N, it performs parallel readout for all columns, has the highest degree of parallelism, and is mainly used for a high-speed image sensor.
FIG. 1 is a structural block diagram showing a structure of a conventional analog readout processing circuit for a solid state image sensor. Referring to FIG. 1, the analog readout processing circuit comprises an analog readout preprocessing circuit and an analog-to-digital converter (ADC). The analog readout preprocessing circuit plays an important role therein. An output voltage of the image sensor is de-noised, amplified, level shifted, and converted from single-end to differential by the analog readout preprocessing circuit and is output to the ADC in a differential structure. A sampling preprocessing circuit in the ADC outputs a differential signal, quantizes the differential signal, and temporarily stores the quantized value in a register, which is prepared for outputting at an input/output (IO) port directly or after being processed in a digital domain.
In the prior art, the analog readout preprocessing circuit only samples and converts a reset signal and an effective light intensity signal output from the image sensor one time, resulting in a large readout noise and a longer analog readout time, wherein the output noise of the analog read preprocessing circuit contains a noise from the image sensor.
The analog readout preprocessing circuit according to the present disclosure comprises an extended count-type integrating circuit which can effectively reduce a thermal noise from the image sensor and circuits by integrating the reset signal and the light intensity signal output from the image sensor many times, thereby providing a signal readout with high performance.
In carrying out the present disclosure, the inventor has found that the use of the extended count-type integrating circuit requires multiple sampling of the signal, thereby increasing the readout time and lowering a readout speed, which is unfavorable for the application of the readout circuit in cases where a high speed is needed.